U.S. Pat. No. 4,700,668 discloses a chain saw driven by a two-stroke engine having a pneumatically-actuated injection pump. The pneumatic drive chamber of the injection pump is partitioned by a membrane into a pulse chamber and a return chamber. The pulse chamber is charged directly with the crankcase pressure while the return chamber communicates with the atmosphere. The crankcase pressure alternates with the upward and downward movement of the piston and thereby moves the membrane up and down which, in turn, moves a pump piston attached to the membrane. The pump piston is guided in a pump cylinder and delimits a pumping chamber. The pump piston moves into the pumping chamber and pumps fuel via an outlet valve configured as a check valve. With the subsequent return movement of the pump piston, the check valve closes and an intake valve admits fuel into the pumping chamber for the next injection cycle. The intake valve opens in the direction of the pumping chamber and is configured as a check valve.
Such injection pumps are attached to the two-stroke engine itself in order that the injection lines be rigid and be as short as possible so that a precisely timed ignition is assured.
During operation, the two-stroke engine becomes hot especially in the region of the cylinder but also at the crankcase. All components mounted on the cylinder and the crankcase including the flange-connected injection pump are heated in this manner. In the hot injection pump, the fuel conducted therethrough and its easily boilable components tend toward evaporation because of the heat and vapor bubbles are formed. These vapor bubbles disturb the correct metering of fuel because the injection pump can only pump a defined volume in view of the way it has been constructed. The appropriate quantity of fuel does not reach the injection valve if a vapor bubble is drawn in and pumped to the injection valve. Large vapor bubbles cause the engine to die.
To provide an undisturbed operation of the fuel injection engine, the injection pump must therefore be thermally insulated in order to avoid the injection pump from becoming hot. However, insulating material between the engine and the injection pump would take up too much building space in order to be adequate.
In addition, the considerable vibrations of the two-stroke engine lead to operational disturbances in the injection pump and especially with respect to its check valves.